Solar clock and school start time effects on adolescents' chronotype and sleep: A review of a gap in the literature.

Circadian rhythms are entrained by external factors such as sunlight and social cues, but also depend on internal factors such as age. Adolescents exhibit late chronotypes, but worldwide school starts early in the morning leading to unhealthy sleep habits. Several studies reported that adolescents benefit from later school start times. However, the effect of later school start time on different outcomes varies between studies, and most previous literature only takes into consideration the social clock (i.e. local time of school starting time) but not the solar clock (e.g. the distance between school start time and sunrise). Thus, there is an important gap in the literature: when assessing the effect of a school start time on chronotype and sleep of adolescents at different locations and/or seasons, the solar clock might differ and, consistently, the obtained results. For example, the earliest school start time for adolescents has been suggested to be 08:30 hours, but this school start time might correspond to different solar times at different times of the year, longitudes and latitudes. Here, we describe the available literature comparing different school start times, considering important factors such as geographic position, nationality, and the local school start time and its distance to sunrise. Then, we described and contrasted the relative role of both social and solar clocks on the chronotype and sleep of adolescents. As a whole, we point and discuss a gap in literature, suggesting that both clocks are relevant when addressing the effect of school start time on adolescents' chronotype and sleep.

A better alignment between chronotype and school timing is associated with lower grade retention in adolescents.

Schools start early in the morning all over the world, contrasting with adolescents' late chronotype. Interestingly, lower academic performance (i.e. grades or qualifications) was associated with later chronotypes. However, it is unclear whether it is a direct effect of chronotype or because students attend school too early to perform at their best. Moreover, little is known about how this affects students' academic success beyond their grades. To address this gap in knowledge, we studied how school timing and chronotype affect grade retention (i.e. repeat a year) in a unique sample of students randomly assigned to one of three different school timings (starting at 07:45, 12:40, or 17:20). Even when controlling for academic performance, we found that later chronotypes exhibit higher odds of grade retention only in the morning, but not in later school timings. Altogether, ensuring a better alignment between school timing and students' biological rhythms might enhance future opportunities of adolescents.

Chronotype at the beginning of secondary school and school timing are both associated with chronotype development during adolescence.

The misalignment between late chronotypes and early school start times affect health, performance and psychological well-being of adolescents. Here we test whether, and how, the baseline chronotype (i.e. chronotype at the beginning of secondary school) and the school timing affect the magnitude and the direction of the developmental change in chronotype during adolescence. We evaluated a sample of Argentinian students (n = 259) who were randomly assigned to attend school in the morning (07:45 a.m.-12:05 p.m.), afternoon (12:40 p.m.-05:00 p.m.) or evening (05:20 p.m.-09:40 p.m.) school timings. Importantly, chronotype and sleep habits were assessed longitudinally in the same group of students along secondary school (at 13-14 y.o. and 17-18 y.o.). Our results show that: (1) although chronotypes partially align with class time, this effect is insufficient to fully account for the differences observed in sleep-related variables between school timings; (2) both school timing and baseline chronotype are independently associated with the direction and the magnitude of change in chronotype, with greater delays related to earlier baseline chronotypes and later school timings. The practical implications of these results are challenging and should be considered in the design of future educational timing policies to improve adolescents' well-being.

Background music changes the policy of human decision-making: Evidence from experimental and drift-diffusion model-based approaches on different decision tasks.

Music is ubiquitous in our lives. Although we listen to music as an activity in and of itself, music is frequently played while we are engaged in other activities that rely on decision-making (e.g., driving). Despite its ubiquity, it remains unknown whether and how background music modulates the speed and accuracy of decision-making across different domains. We hypothesized that music could affect decision-making through a subjective-timing distortion or via a policy shift toward less-cautious responding. We analyzed response times and accuracy from more than 100-thousand decisions and mapped the effects of music onto decision-process components with a mechanistic model of decision-making. We found evidence supporting the latter hypothesis, by which decisions-across domains-were faster but less accurate with music, and this trade-off was mainly driven by a less conservative decision policy. Overall, our results suggest that background music shapes our decisions by making us less cautious. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Effects of lockdown on human sleep and chronotype during the COVID-19 pandemic.

The COVID-19 pandemic [1] resulted in many countries imposing a lockdown, which in turn reduces sunlight exposure and alters daily social schedules. Since these are the main entrainment factors for biological rhythms [2], we hypothesized that the lockdown may have affected sleep and circadian rhythms. We indeed show that participants slept longer and later during lockdown weekdays, and exhibited lower levels of social jetlag. While this may seem to be an overall improvement of sleep conditions, chronotype was also delayed under the lockdown. This signature of a weaker light-dark cycle should be monitored attentively since it may progressively cause disruptive effects on sleep and circadian rhythms, affecting human performance and health [3].

Time drawings: Spatial representation of temporal concepts.

Time representation is a fundamental property of human cognition. Ample evidence shows that time (and numbers) are represented in space. However, how the conceptual mapping varies across individuals, scales, and temporal structures remains largely unknown. To investigate this issue, we conducted a large online study consisting in five experiments that addressed different time scales and topology: Zones of time, Seasons, Days of the week, Parts of the day and Timeline. Participants were asked to map different kinds of time events to a location in space and to determine their size and color. Results showed that time is organized in space in a hierarchical progression: some features appear to be universal (i.e. selection order), others are shaped by how time is organized in distinct cultures (i.e. location order) and, finally, some aspects vary depending on individual features such as age, gender, and chronotype (i.e. size and color).

Time to decide: Diurnal variations on the speed and quality of human decisions.

Human behavior and physiology exhibit diurnal fluctuations. These rhythms are entrained by light and social cues, with vast individual differences in the phase of entrainment - referred as an individual's chronotype - ranging in a continuum between early larks and late owls. Understanding whether decision-making in real-life situations depends on the relation between time of the day and an individual's diurnal preferences has both practical and theoretical implications. However, answering this question has remained elusive because of the difficulty of measuring precisely the quality of a decision in real-life scenarios. Here we investigate diurnal variations in decision-making as a function of an individual's chronotype capitalizing on a vast repository of human decisions: online chess servers. In a chess game, every player has to make around 40 decisions using a finite time budget and both the time and quality of each decision can be accurately determined. We found reliable diurnal rhythms in activity and decision-making policy. During the morning, players adopt a prevention focus policy (slower and more accurate decisions) which is later modified to a promotion focus (faster but less accurate decisions), without daily changes in performance.

Suprachiasmatic astrocytes modulate the circadian clock in response to TNF-α.

The immune and the circadian systems interact in a bidirectional fashion. The master circadian oscillator, located in the suprachiasmatic nuclei (SCN) of the hypothalamus, responds to peripheral and local immune stimuli, such as proinflammatory cytokines and bacterial endotoxin. Astrocytes exert several immune functions in the CNS, and there is growing evidence that points toward a role of these cells in the regulation of circadian rhythms. The aim of this work was to assess the response of SCN astrocytes to immune stimuli, particularly to the proinflammatory cytokine TNF-α. TNF-α applied to cultures of SCN astrocytes from Per2(luc) knockin mice altered both the phase and amplitude of PER2 expression rhythms, in a phase-dependent manner. Furthermore, conditioned media from SCN astrocyte cultures transiently challenged with TNF-α induced an increase in Per1 expression in NIH 3T3 cells, which was blocked by TNF-α antagonism. In addition, these conditioned media could induce phase shifts in SCN PER2 rhythms and, when administered intracerebroventricularly, induced phase delays in behavioral circadian rhythms and SCN activation in control mice, but not in TNFR-1 mutants. In summary, our results show that TNF-α modulates the molecular clock of SCN astrocytes in vitro, and also that, in response to this molecule, SCN astrocytes can modulate clock gene expression in other cells and tissues, and induce phase shifts in a circadian behavioral output in vivo. These findings suggest a role for astroglial cells in the alteration of circadian timing by immune activation.

Diurnal variation in endotoxin-induced mortality in mice: correlation with proinflammatory factors.

Many immune parameters exhibit daily and circadian oscillations, including the number of circulating cells and levels of cytokines in the blood. Mice also have a differential susceptibility to lipopolysaccharide (LPS or endotoxin)-induced endotoxic shock, depending on the administration time in the 24 h light-dark (LD) cycle. We replicated these results in LD, but we did not find temporal differences in LPS-induced mortality in constant darkness (DD). Animals challenged with LPS showed only transient effects on their wheel locomotor activity rhythm without modification of circadian period and phase. Levels of several key factors involved in the pathology of sepsis and septic shock were tested in LD. We found that LPS-induced levels of interleukin (IL)-1beta, IL-6, JE (MCP-1), and MIP1alpha were significantly higher at zeitgeber time (ZT) 11 (time of increased mortality) than at ZT19 (ZT12 = time of lights-off in the animal quarters for the 12L:12D condition). Our results indicate that the differences found in mortality that are dependent on the time of LPS-challenge are not directly related to an endogenous circadian clock, and that some relevant immune factors in the development of sepsis are highly induced at ZT11, the time of higher LPS-induced mortality, compared to ZT19.

Circadian modulation of gene expression, but not glutamate uptake, in mouse and rat cortical astrocytes.

BACKGROUND: Circadian clocks control daily rhythms including sleep-wake, hormone secretion, and metabolism. These clocks are based on intracellular transcription-translation feedback loops that sustain daily oscillations of gene expression in many cell types. Mammalian astrocytes display circadian rhythms in the expression of the clock genes Period1 (Per1) and Period2 (Per2). However, a functional role for circadian oscillations in astrocytes is unknown. Because uptake of extrasynaptic glutamate depends on the presence of Per2 in astrocytes, we asked whether glutamate uptake by glia is circadian. METHODOLOGY/PRINCIPAL FINDINGS: We measured glutamate uptake, transcript and protein levels of the astrocyte-specific glutamate transporter, Glast, and the expression of Per1 and Per2 from cultured cortical astrocytes and from explants of somatosensory cortex. We found that glutamate uptake and Glast mRNA and protein expression were significantly reduced in Clock/Clock, Per2- or NPAS2-deficient glia. Uptake was augmented when the medium was supplemented with dibutyryl-cAMP or B27. Critically, glutamate uptake was not circadian in cortical astrocytes cultured from rats or mice or in cortical slices from mice. CONCLUSION/SIGNIFICANCE: We conclude that glutamate uptake levels are modulated by CLOCK, PER2, NPAS2, and the composition of the culture medium, and that uptake does not show circadian variations.

Suprachiasmatic astrocytes as an interface for immune-circadian signalling.

The hypothalamic suprachiasmatic nuclei (SCN), the site of a mammalian circadian clock, exhibit a dense immunoreactivity for glial fibrillary acidic protein (GFAP), a specific marker for astrocytes. Although there is evidence of a circadian variation in GFAP-IR in the hamster SCN and of the participation of glial cells in input and output mechanisms of the clock, the role of these cells within the circadian system is not clearly understood. The fact that astroglia can express and respond to cytokines suggests that they could work as mediators of immune signals to the circadian system. In the present study, we have found a daily variation of GFAP-IR in the mouse SCN, peaking during the light phase. In addition, we have identified GFAP and nuclear factor-kappaB (NF-kappaB) in glial cells within the SCN and in primary cultures of the mouse SCN. Moreover, SCN glia cultures were transfected with an NF-kappaB/luc construct whose transcriptional activity was increased with lipopolysaccharide 2 mug/ml, tumor necrosis factor-alpha 20 ng/ml, or interleukin-1alpha 100 ng/ml, after 12 hr of stimulation. These results suggest that the glial cells of the SCN can mediate input signals to the mouse circadian system coming from the immune system via NF-kappaB signaling.